1. Field of this Invention
This invention relates to a manufacturing method of a bonded wafer, and in particular, to a manufacturing method of a bonded wafer using an ion implantation delamination method.
2. Description of the Related Art
An ion implantation delamination method has been known as a manufacturing method of SOI (Silicon On Insulator) wafer, which is an example of the manufacturing method of a bonded wafer. This method is performed in this way: hydrogen is ion implanted on a bonding surface (hereinafter, referred to as a first main surface) of a bond wafer so that a delamination ion implantation layer, which is a high concentration hydrogen layer, is formed at a predetermined depth, a base wafer is bonded to the first main surface and thereafter, the bond wafer is delaminated off at the delamination ion implantation layer, which is also referred to as a Smart-Cut process (“Smart Cut” is a registered trade mark): (for example, see JP No. 3048201). On the other hand, it is actually enabled to insert, a polysilicon layer or an amorphous silicon layer as an additional function layer right under or right over of an buried oxide film of an SOI wafer for, for example, heavy metal gettering. In order to incorporate the additional function layer in the process of the ion implantation delamination method, it is thought that the following change in the process is necessary. That is, after a buried oxide layer is formed on the bond wafer side, an additional function layer is further formed by a chemical vapor deposition (CVD for short) method or the like. Then, hydrogen is ion implanted so that hydrogen reaches into the bonded wafer from the additional function layer side to form the delamination ion implantation layer. Next, the additional function layer on the bonded wafer is bonded to a base wafer, the bond wafer is delaminated off at the delamination ion implantation layer to thereby obtain an SOI layer as a remaining bonded semiconductor thin layer.
Since, in an ion implantation delamination method, the following technique has been known on a dosage of ion implantation for delamination in a bond wafer. That is, energy of ion implantation has to be adjusted in order to regulate a position (depth) of formation of an delamination ion implantation layer from the first main surface, depending on a required film thickness of an SOI layer. With a shallower position of formation of the delamination ion implantation layer, a critical dosage required for delamination is smaller. On the other hand, a surface roughness of a delamination surface is affected by a dosage of ion implantation and with a smaller dosage, a surface roughness of the delamination surface is smaller. Therefore, in a case where a position of formation of a delamination ion implantation layer is shallower, a dosage of ion implantation is set lower; with a smaller dosage of ion implantation, a surface roughness is also smaller and if a thin SOI layer is formed, film thickness uniformity of the SOI layer is improved (for example, see page 4 of JP-A 2004-63730).
In a manufacturing method of a bonded wafer, in which a manufacturing method of the SOI wafer is described as an example, necessity arises for polishing for planarization of a surface of an additional function layer in order to perform good bonding to a base wafer in a case where an additional function layer is incorporated. If inclination occurs on a polished surface of the additional function layer in the planarization polishing, a film thickness of the additional function layer is uneven and a depth of a hydrogen ion implantation into a bond wafer is adversely affected by the non-uniformity of the film thickness of the additional function layer and a problem arises that leads to non-uniformity of a film thickness of a bonded semiconductor thin film (equivalent to an SOI layer) obtained by ion implantation delamination.
In addition, ion implantation of hydrogen is applied from the additional function layer side into a bond wafer; therefore, a problem arises that leads to an adverse influence of a surface roughness of a polished surface of an additional function layer exerted on a surface roughness of a delamination surface of a delamination ion implantation layer.
Furthermore, ion implantation of hydrogen is applied through an additional function layer, which leads to a problem that the ion implantation requires a great amount of energy. Hence, a depth of a delamination ion implantation layer is limited to a position not relatively deep in a bond wafer: it is difficult to form the ion implantation layer at a position equal to or deeper than, for example, 1 μm from a polished surface of the additional function layer.
A method to solve the problems is that an additional function layer is deposited on a base wafer side and the layer is bonded to a bond wafer with an oxide film interposed therebetween. With such a method adopted, however, since a bonding surface is closer to an SOI layer, in which a device is fabricated, new problems arise such that an impurity incorporated onto a bonding interface adversely affects a device layer, and that in a case where a thin buried oxide film (for example, 100 nm or less) is required, poor bonding is easy to occur since a surface of the thin oxide film is used as the bonding surface, leading to degradation of a manufacturing yield.
As another solution, a method has been available that hydrogen ion implantation is performed before an additional function layer is deposited on the bond wafer side, however since growth temperature required for deposition of an additional function layer is usually 400° C. or higher, another problem arises that delamination occurs during deposition in the vicinity of the surface of a wafer.
It is a first object of this invention to provide a manufacturing method of a bonded wafer that by conducting two steps of ion implantation forming a delamination ion implantation layer, having a deposition step of an additional function layer therebetween, no adverse influence of non-uniformity of a film thickness of an additional function layer, if it occurs, acts on uniformity of a film thickness of a bonded semiconductor thin layer.
It is a second object of this invention to provide a manufacturing method of a bonded wafer that can form a delamination ion implantation layer with small energy at a more precise position.
It is a third object of this invention to provide a manufacturing method of a bonded wafer that can reduce a surface roughness of the delamination surface of a delamination ion implantation layer.